BME 501 Advanced Topics in Biomedical Systems Spring 2014 Dr. KayBME 501 Lecture Notes – Apr 9 Electrocardiogram (ECG/EKG) • Principles of EKG • Effects of Drugs on EKGs • Arrhythmia and Arrhythmic MechanismsSA Node AV Node Electrical Signal Through Heart • Wave of depolarization travels through heart • Starts in atria • Moves to ventricles • Depolarization followed by repolarization in each section of heart• Can detect electrical activity of heart by putting electrodes on a subject’s chest • Body surface manifestations of depolarization and repolarization waves of heart recorded • Signal has distinct features as electrical impulse moves through heart • EKG represents the summed electrical activity of the cardiac muscle fibers Electrocardiogram (ECG/EKG)• APs travel through heart following initiation in SA node • APs are separated temporally and spatially in heart • Summation of depolarization and repolarization produce characteristic EKG Electrocardiogram (ECG/EKG)Electrocardiogram (ECG/EKG) • P-wave: marks atrial depolarization – Small, broad wave – Coincides with upstrokes of thousands of right and left atrial APs – Less than 0.1 seconds in length • PR interval: marks AV node delay – Interval between start of P-wave and start of QRS complex (technically, PQ interval) – Atrial contraction occurs during this period – Represents time taken for excitation to travel through atria, AV node, and His-Purkinje system• PR interval: marks AV node delay – Mostly comprised of delay in transmission through AV node (allows atrial systole to precede ventricular systole and filling of ventricles) – Should not exceed 0.2 seconds – Longer PR intervals indicate defect in conduction pathway: heart block – EKG isoelectric for most of PR interval duration (due to insulating annulus fibrosus) – Atrial repolarization not registered on EKG because it is asynchronous and slow, does not generate detectable extracellular current (not because it is obscured by QRS complex) Electrocardiogram (ECG/EKG)• QRS complex: marks ventricular excitation – Rapid depolarization of large mass of ventricular muscle produces large deflection – Q-wave: initial downward spike; R-wave: upward spike; S-wave: second downward spike – All 3 components not necessary present in all records – Normally lasts up to 0.1 seconds – Longer/wider QRS complexes indicate ventricular ectopic beat or block in one of the bundle branches – Bundle branch block: R-wave has characteristic ‘M’-shaped notch due to staggered activation of ventricle – VAT: ventricular activation time Electrocardiogram (ECG/EKG)• ST segment: marks plateau of ventricular AP – EKG isoelectric: ventricle uniformly depolarized, no extracellular current flowing – Ischemic heart disease causes displacement of ST segment due to “injury currents”  ST segment depressed in angina  ST segment elevated in transmural infarct shortly after heart attack Electrocardiogram (ECG/EKG)• T-wave: marks ventricular repolarization – Ventricular repolarization slower and less synchronous than depolarization – Generates broad, asymmetrical wave – T-wave is generally upright (same direction as R-wave) due to ‘cardiac dipole’ in repolarizing cardiac muscle – T-wave inversion can indicate myocardial ischemia Electrocardiogram (ECG/EKG)• 3 electrodes placed on limbs provide electrical information for frontal plane • Electrodes placed on right arm, left arm, and left leg form points of triangle • Leads connecting 3 limb electrodes form sensing triangle around heart: Einthoven’s Triangle - I: Left arm (+) to Right arm (−); horizontal (0°) - II: Left leg (+) to Right arm (−); 60° from horizontal - III: Left leg (+) to Left arm (−); 120° from horizontal Electrocardiogram (ECG/EKG)• 3 electrodes on limbs also used for unipolar limb leads: – Produced by feeding signal from two electrodes simultaneously into (−) terminal of voltmeter and remaining electrode to (+) terminal – Left arm to (+) terminal = aVL (−30° from horizontal) – Right arm to (+) terminal = aVR (−150° from horizontal) – Foot to (+) terminal = aVF (+90° from horizontal) Electrocardiogram (ECG/EKG)• Additional 6 precordial electrodes (V1−V6) placed across chest for connection to (+) terminal for 6 separate recordings • All 3 limb electrodes connected to (−) terminal to produce a mid-chest reference point • Electrodes encircle heart and “look” transversely across chest • Useful because wave of excitation travels in 3 dimensions, not just in frontal plane Electrocardiogram (ECG/EKG)• V1 and V2 – Placed over fourth intercostal space, immediately to the right and left of sternum (respectively) – Record right ventricular activity best • V3 and V4 – V3 placed between V2 and V4 – V4 placed over fifth intercostal space in mid-clavicular line – Record interventricular septal activity best • V5 and V6 – Placed at same level as V4, in anterior axillary and mid-axillary lines (respectively) – Record left ventricular activity best Electrocardiogram (ECG/EKG)• At any instant during spread of excitation through heart – Resting zone with diffuse cloud of (+) extracellular charges – Excited zone with diffuse cloud of (−) extracellular charges • Diffuse charges can be represented as a single charge at its electrical center (pole) • During excitation, heart can be represented by one (+) pole and one (−) pole • Size of associated EKG deflection depends on magnitude of dipole and orientation of dipole relative to recording lead Electrocardiogram (ECG/EKG)• Dipole is surrounded by positive and negative potential fields • Fields grow weaker with increasing distance (like fields around a magnet) • When leads of voltmeter are aligned with the two poles, difference between positive and negative potential fields is recorded optimally • When leads are at right (90°) angles to dipole, they detect no potential difference Electrocardiogram (ECG/EKG)• Ventricular dipole swings counter-clockwise in frontal plane during depolarization: • First part of ventricles to depolarize is left side of interventricular septum (by left bundle branch) – Dipole magnitude small, ~120° from horizontal (down and right) • Next, remaining septum and most of subendocardium depolarize – Bulky left ventricle dominates – Dipole magnitude large, ~60° from horizontal